Cellular ceramics are materials with a high level of porosity that exhibit an enclosed empty space with faces and solid edges. The faces can be fully solid to give a closed cell material or faces can be voids to give an open cell material. Pores can reside in the cell walls, or the material can have a mixed morphology of closed and open cells. Cellular ceramics can display a unique combination of properties, including: high temperature and environmental stability; low density: low thermal conductivity; low dielectric constant; low thermal mass; high specific strength; high permeability; high thermal shock resistance; high porosity; high specific surface area; high wear resistance; high resistance to chemical corrosion; and high tortuosity of flow paths. These properties make them highly valuable for various engineering applications including: filtration of molten metals or of particulates from exhaust gases; radiant burners; catalyst supports; biomedical devices; kiln furniture; reinforcement for metal or polymer matrix composites; bioreactors; fiber-free thermal management components; supports for space mirrors; lightweight sandwich structures; heat sinks; electrodes; heat exchangers: and components in solid oxide fuel cells (SOFC).
The properties stem from the porosity where a specific material property can be achieved by the choice of material and the fabrication method employed to achieve the porosity. The fabrication method influences the type of cell, the size and shape distribution of the cells, and the interconnectivity of the cells and can limit the shape and size of the ceramic part. Performance and properties depend on the macro- and microstructure of the cellular ceramic component. The nature and distribution of the cell walls influences permeability and strength. Compositional purity affects chemical and oxidation resistance, high-temperature creep, electrical resistivity and thermal properties. The compositional purity also depends strongly on the processing method. The fabrication method also has a large influence on the cost of the material and some are only suited toward high performance, high added value products. Generally, three different methods have been used to form most ceramic foams: replication of a sacrificial foam template; direct foaming of a slurry; and pyrolysis of fugitive pore formers.
The majority of ceramic foams are fabricated industrially using a replication process to form reticulated ceramics with open-cells of voids surrounded by a web of ceramic struts. The replication process was the first developed for manufacture and produces a macroporous ceramic body (see Schwartzwalder el al. U.S. Pat. No. 3,090,094). This process involves, impregnating a flexible polymeric foam with ceramic slurry, removing excess slip by squeezing or centrifuging, drying with the burn-out of the polymer template and finally sintering at high-temperature. The organic foam must possess reproducible properties, such as shape memory upon squeezing, limited tolerances of cell size and size distribution, and must completely burn-out during sintering. The ceramic slurry can employ a wide variety of oxides and non-oxides and can include binders, rheological agents and/or setting compounds to facilitate coating and/or improve adherence of the ceramic particles to the polymer template. The ceramic struts are generally hollow.
A uniform unfired coating on the polymer template and complete removal of excess ceramic slip before firing is critical to avoid closed cells in the final porous ceramic. Firing must be conducted at an appropriate rate during polymer burn-out to avoid creating stresses and large defects in the ceramic. During heating, the expansion and gas evolution of the polymer can generate stresses that damage the ceramic coating if heating is not carefully controlled. Ceramic struts containing macroscopic flaws are often observed in commercially available cellular ceramics.
The direct foaming method involves generation of bubbles inside a slurry of ceramic powders or inside a ceramic precursor solution to create a foam which is subsequently set to maintain a porous morphology and finally sintered at high temperature. The foaming agent is a volatile liquid, such as a low boiling point solvent, a decomposable solid, such as CaCO3 powder, or a gas that is generated in situ by chemical reactions, such as that generated during thermal decomposition of a silicone resins, oxidation of a solid C or SiC filler to form CO2, or can be added by gas injection.
The foam's morphology depends on concurrent development and stability of the liquid foams. Drainage of the liquid or suspension through the cell edges occurs until an equilibrium state is reached. The foam is coarsened by gas diffusion among bubbles leading to a relatively large dispersion of cell sizes as well as to the increasing of the average cell size with any delays during processing. Ultimately the liquid film can rupture and the foam can collapse. To avoid collapsing of the liquid foam and loss of the cellular morphology, special additives are included to harden the foamed structure once it is stabilized. Setting strategies that have been used include polymerization of an organic monomer to stabilize foams from aqueous ceramic powder suspensions, clotting of protein or ovalbumin binders, gelling due to enzymes, starch, cellulose derivatives or alginates, cross-linking of polyurethane precursors, or freezing. For example, Wood et al, U.S. Pat. No. 3,833,386 use polyurethane network precursors mixed with an excess of aqueous ceramic slurry which is foamed and then sintered at high temperature to produce the cellular ceramic. Surfactants can be used to achieve control of the foam structure. The ceramic foams obtained by direct foaming are ultimately dried and sintered by conventional means.
Foams formed by direct foaming differ from foams obtained by a replica technique in that both closed and open-cell form, generally a wider range of cell dimensions results, cell dimensions are generally limited to smaller cells due to liquid foam stability limitations, and wider range of porosity can be achieved. These materials typically display cell walls containing interconnecting pores which leads to a different permeability behavior than those displayed by reticulated foams that allow a finer adjustment of fluid transport within the structure. Ceramic struts are dense and possess a limited amount of defects which lead to improved mechanical strength. With direct foaming techniques, one can produce many shapes of the final ceramic part without subsequent machining. Cellular materials produced by direct foaming can display unwanted anisotropy in the structure, due to differing expansion in the rise direction versus the lateral directions during foaming.
Burn-out or decomposition of fugitive pore formers is another method for producing cellular ceramics. Hollow cells are produced when the solid material that occupies volume within the mixture decomposes during heating at high temperature. Starch, wax, polymeric beads, for example, poly(methyl methacrylate), polystyrene, and poly(vinyl chloride), carbon black, and sawdust have all been used as fugitive pore formers. Pore size and shape is controlled by the characteristics of the sacrificial filler. Graded structures can be obtained by layering fillers of varying dimensions. In order to produce a highly porous cellular structure, a large volume of the porogen agent must be mixed with the ceramic phase which leads to the development of a large amount of gas during sintering and can lead to the formation of cracks in the ceramic body. Fugitive pore formers can lead to cellular ceramics and form closed or open cells depending on the volume fraction of pore formers and the nature of gas generation.
In addition to the porous ceramic body, a dense ceramic surface layer may be desired in order to obtain a maximum mechanical strength and/or to enable a directed fluid flow through the ceramic object. The formation of a dual structure, where a dense layer, for example a dense surface, yields to a porous structure without a discontinuous interface between the porous and dense layers has been prepared with few processing steps. For example, Miura et al. U.S. Pat. No. 4,670,304 discloses the impregnation of a foam with a ceramic slurry, followed by centrifugation of the impregnated foam to force a portion of the slurry to the exterior of the foam, followed by drying, baking and sintering to form a dual structure. The preparation of a dual structure ceramic without steps of impregnating and centrifugation would be advantageous for devices employing a dual structured ceramic.